Since 1980s the CDMA technique has been used in a digital mobile communication field and becomes a vital mobile communication mode with the high spectrum availability and powerful anti-interference capability. At present, people pay more attention on the third generation mobile communication system and have done a lot of work on it such as Future Public Land Mobile Telecommunication System, i.e. IMT-2000 or Universal Mobile Telecommunication System (UMTS). It is a common sense that the CDMA techniques will be deployed on the third generation communication system. Among the CDMA techniques the WCDMA is an important air interface scheme and is greatly concerned.
In the present WCDMA protocol, each cell has its own clock, but there is no unique reference time among cells. Therefore, relative timing relationship of different channels in a cell is definite, but timing relationship among cells is unknown. This means that every channel within a NodeB is synchronous, but channels among different NodeBs are asynchronous. In other words, User Equipment (UE) knows timing of the signals in its own cell but does not know any timing information about other cells. Problems arise in this situation.
1) Timing drift among different NodeBs for a long time may cause overlaps of downlink synchronization channels so that the downlink synchronization channels are unable to be differentiated, and the UE is impossible to find a adjacent cell so that a handover can not be made. Since the UE has no timing information of other cells, it is necessary to search the adjacent cells in all phases; the result is that resource is wasted, design is more difficult and at the same time power consumption of the UE is great so that its await time is shorten.
2) During soft handover, the UE must measure the arriving time differences of the signals from adjacent cells in order to ensure that delay-time of the downlink traffic channels among cells in active sets are minimum to take advantage for combination. Nevertheless, asynchronies among cells make the UE more complicated; the arriving time of multipath signals is difficult to be measured accurately and is varied as propagation environment. After the UE has reported upward the arriving time differences of signals from different cells, the target NodeB making soft handover must adjust the transmitting moment of the downlink traffic channel to obtain a minimum delay-time difference for the traffic channels. For supporting the soft handover and downlink transmitting moment adjustment, a complicated signaling procedure must be introduced; this not only increases the communication between Radio Network Controller (RNC) and NodeB, but also decreases the handover speed, success and reliability, which affect the system capacity and service quality.